1. Field of the Invention
The present invention relates to an ink-jet recording apparatus, and more particularly, to a technique for controlling the driving of an ink-jet head provided in an ink-jet recording apparatus.
2. Related Art
An ink-jet head which expels ink droplets by inducing variations in the volume of ink through use of a heating element has been known as an ink-jet head to be used with an ink-jet recording apparatus such as an ink-jet printer. Another known type of ink-jet head is one which expels ink droplets by changing the volume of an ink chamber (a pressure generation chamber) in communication with an ink nozzle. A vibrating plate is formed in part of a circumferential wall so as to become resiliently deformable in an outward direction relative to the surface of a circumferential wall with which an ink chamber is partitioned, as well as to correspond to one dot. Ink droplets are expelled expelled from the ink nozzle in communication with the ink chamber by application of a drive signal to a piezoelectric vibrator (or a pressure generating element) mounted on the vibrating plate to thereby vibrate the vibrating plate.
Of the foregoing ink-jet heads, an ink-jet head which expels ink droplets by changing the volume of the ink chamber through use of the piezoelectric vibrator is classified into known types, i.e., an ink-jet head of catapult-like expelling type and an ink-jet head of pressurized expelling type. In the ink-jet head of catapult-like expelling type, by actuation of a piezoelectric vibrator, a vibrating plate constituting part of the ink chamber is displaced beforehand in a direction in which positive pressure develops in the ink chamber as a result of a reduction in its volume. In this state, the vibrating plate is displaced in a direction in which negative pressure develops in the ink chamber as a result of an increase in its volume, as a result of which an ink meniscus is withdrawn into a discharge port. At the instant when the ink meniscus is withdrawn to the innermost position, the vibrating plate is displaced in the reverse direction, thus expelling an ink droplet from the ink nozzle.
In contrast, in the ink-jet head of pressurized expelling type, after having been displaced in a direction in which negative pressure develops in the ink chamber as a result of an increase in the volume of the ink chamber, the vibrating plate is displaced in a direction in which positive pressure develops in the ink chamber as a result of a decrease in the volume of the ink chamber the instant when the ink meniscus formed in the ink discharge port becomes stationary, thus expelling an ink droplet. To perform expelling operations such as those set forth, a drive control section applies, between electrodes of the piezoelectric vibrator, a drive signal Vnco having a waveform, such as that shown in FIG. 19A. In the drive signal Vnco, a period from time t21 to time t31 corresponds to one drive cycle. Immediately before commencement of a current drive operation (i.e., during a period from time t19 to time t21), the vibrating plate is displaced in a direction in which the volume of the ink chamber decreases, by application of a first drive voltage V1 to the piezoelectric vibrator. In a case where an ink droplet is expelled by means of the current driving operation, the voltage applied to the piezoelectric vibrator is changed from the first drive voltage V1 to a second drive voltage V2 during a period from time t21 to time t22, thus displacing the vibrating plate in a direction in which the volume of the ink chamber increases. This state is maintained for a period from time t22 to time t23. Subsequently, the voltage applied to the piezoelectric vibrator is changed from the second drive voltage V2 to a third drive voltage V3 during a period from time t23 to time t24, thus displacing the vibrating plate in a direction in which the volume of the ink chamber decreases. Such a state is maintained for a period from t24 to time t25, and an ink droplet is expelled from the ink nozzle. Subsequently, during a period from time t25 to time t26, the voltage applied to the piezoelectric vibrator is changed from the third drive voltage V3 to an intermediate voltage (Vm) between the first and second drive voltages V1 and V2 or between the third drive voltage V3 and the second drive voltage V2, to thereby displace the vibrating plate in a direction in which the volume of the ink chamber is increased to its intermediate level, thus interrupting the flow of ink by means of the ink discharge port. After expelling of ink, the voltage applied to the piezoelectric vibrator is maintained at an intermediate voltage Vm during a period from time t26 to time t28 without regard to whether or not ink is expelled during the next drive cycle, whereby the volume of the ink chamber is maintained at its intermediate level. Immediately before commencement of the next drive operation (during a period from time t28 to time t29), the voltage applied to the piezoelectric vibrator is changed from the intermediate voltage Vm to the first drive voltage V1, thus displacing the vibrating plate in a direction in which the volume of the ink chamber decreases. Such a state is maintained for a period from time t29 to time 31.
In contrast, in a case where the expelling of an ink droplet from the ink nozzle is suspended for the period of time corresponding to one dot during the current drive cycle, a drive signal Vnco having a waveform such as that shown in FIG. 19B is applied between the electrodes of the piezoelectric vibrator. Immediately before commencement of the current drive operation (during a period from time t18 to time t21), the vibrating plate is displaced in a direction in which the volume of the ink chamber decreases from its intermediate level by means of the first drive voltage V1, as in the case described by reference to FIG. 19A in which an ink droplet is expelled during the current drive operation. During a period from time t21 to time 31 at which the next drive operation is commenced, the volume of the ink chamber is maintained at a reduced value by application of the first drive voltage V1 to the piezoelectric vibrator.
Here, the first and third drive voltages V1 and V3 may be respectively set to different potentials or to an identical potential as in the case such as that shown in FIG. 19A.
However, in a case where each vibrating plate is driven as has been conventionally practiced, if the piezoelectric vibrator is driven according to print data without consideration of the result of a previous driving operation or of a drive history such as the next driving conditions, neither improvement of print quality nor higher speed operation can be accomplished.
Specifically, in order to expel ink droplets from an ink nozzle, the vibrating plate is displaced during a period from time t12 to time 13 during the previous drive cycle until the ink nozzle becomes prepared for a squiring operation. The vibrating plate is then displaced during a period from time t13 to time t14 in a direction in which the volume of the ink chamber decreases. The vibrating plate is further displaced during a period from time t13 to time t14 in a direction in which the volume of the ink chamber decreases. Subsequent1y, the vibrating plate is displaced during a period from time t15 to time t16 until the volume of the ink chamber reaches an intermediate value. As shown in FIG. 19C, the vibrating plate keeps oscillating even after time t16, and the ink meniscus also oscillates in the same manner. As shown in FIG. 19A, in such a case, if the expelling of the ink droplet is continually carried out, a stable driving operation can be ensured by bringing the movement of the piezoelectric vibrator into synchronization with the movements of the vibrating plate or the ink meniscus. Alternatively, in a state in which the vibration of the vibrating plate or the ink meniscus has become sufficiently stationary, the vibrating plate is displaced in a direction in which the volume of the ink chamber increases, by changing the voltage applied to the piezoelectric vibrator from the first drive voltage V1 to the second drive voltage V2. An ink droplet can be squired stably, so long as the volume of the ink chamber is reduced by changing the voltage applied to the piezoelectric vibrator from the second drive voltage V2 to the third drive voltage V3.
As shown in FIGS. 19B and 19C, after the ink droplet has been expelled during a period from t13 to t15, the volume of the ink chamber is held at an intermediate value for a period from time t16 to time t18. When the vibration of the vibrating plate or the ink meniscus attempts to subside, the vibrating plate is displaced during a period from time t18 to time t21 so as to reduce the volume of the ink chamber, thus resulting in a state in which the expelling of ink is suspended. In such a case, the vibration of the vibrating plate or the meniscus does not subside sufficiently during a period (i.e., a period from time t21 to t31) before the next drive cycle. As a result, the state of the vibrating plate or the ink meniscus during a period from time t29 to time t31 is different from a state in which the vibration of the vibrating plate or the ink meniscus has subsided sufficiently and different from a vibrating state in which ink droplets are expelled continually (i.e., a vibrating state during a period from time t19 to time 21 shown in FIG. 19C). This may result in inappropriate displacement of the vibrating plate or the ink meniscus when an ink droplet is expelled after the suspended state, thus rendering high-quality printing operations impossible. In order for an existing ink-jet printer to prevent a reduction in print quality, which would otherwise be cased by a difference in the state of the vibrating plate or the ink meniscus according to whether the ink droplets have been expelled or the expelling of an ink droplet has been suspended during the previous drive cycle, a sufficient time must be ensured as an interval between ink expelling operations until the vibration of the vibrating plate or the ink meniscus subsides. As a result, the exiting ink-jet printer cannot be expected to accomplish improved printing operations.